The present invention relates to the field of machining of material by cutting (e.g., shaping parts by removing excess material in the form of chips), and more particularly machining of materials by cutting with cryogenically cooled cutting tools.
Numerous references are cited throughout this application, including the endnotes which appear after the Detailed Description of the Invention. Each of those references are incorporated herein by reference with regard to the pertinent portions of the references cited herein.
As used herein, the term “cutting” includes but is not limited to the following operations: turning, boring, parting, grooving, facing, planing, milling, drilling, and other operations which generate continuous chips or fragmented or segmented chips. The term cutting does not include: grinding, electro-discharge machining, ultrasonic cutting, or high-pressure jet erosion cutting, i.e., operations generating very fine chips that are not well defined in shape, e.g., dust or powder.
Cutting hard or difficult to machine materials, as well as high-speed cutting of materials from all groups except the low-melting point group including zinc or polymers, leads to very high levels of energy dissipated at the cutting tool. Table 1 below presents examples of easy and difficult to machine ferrous and non-ferrous metals with their machining responses modified by both composition and thermo-mechanical condition. Materials characterized by the unit power (Pc) of more than 1 hp/in3/minute, unit energy (Ec) of more than 2.7 J/mm3, and/or hardness of more than 30 HRC are considered difficult to machine. In the case of steels and other metals melting above 1400° C., high-speed machining proves difficult even if the hardness level is only 25 HRC.
TABLE 1Examples of Hardness, Power, Energy and Temperature Encountered in Cutting(1)Nominalincrease inAssumedWorkSpecificAssumedMaterial/ChipUnit PowerUnit EnergyDensitySpecific HeatTemperatureMaterials:Hardness:[hp/in3/minute][Joules/mm3][grams/cm3][cal./(gram * K)][deg. K or C.]Magnesium 40-90 HB0.13-0.170.36-0.46Low strength aluminum alloys 30-150 HB0.200.552.70.212306061 - T6 aluminum alloy0.350.962.70.214002024 - T4 aluminum alloy0.461.262.70.21520Soft copper alloys 10-80 HRB0.501.378.90.0940070Cu-30Zn brass0.591.61Copper and harder copper alloys 80-100 HB0.70-0.801.91-2.188.90.09580Steels:AISI 1020 carbon steel150-175 HB0.581.587.80.11440AISI 1020 carbon steel176-200 HB0.671.837.80.11500Carbon, alloy, and tool 35-40 HRC1.153.147.80.11870steels, 40-50 HRC1.203.287.80.11900various hardness 50-55 HRC1.604.377.80.11~1200levels . . . 55-58 HRC2.757.517.80.11>1500Stainless steels, wrought and135-275 HB1.052.87cast, 30-45 HRC1.123.06various hardness150-450 HB1.123.06levels: . . .Precipitation hardening stainlesssteelsSoft grades of cast irons110-190 HB0.551.50Gray, ductile, and malleable190-320 HB1.123.06gradesTitanium alloys250-375 HB1.0-1.92.73-5.184.40.121186->1600Nickel based superalloys200-360 HB2.05.468.90.11>1350Niobium alloys217 HB1.43.82Molybdenum230 HB1.64.3710.20.061710Tantalum210 HB2.256.14Tungsten320 HB2.36.2819.20.032440Notes:1. Unit power - power at cutting tool required to remove work material at the rate of 1 in3/minute.2. Unit energy - total energy dissipated by cutting tool removing 1 mm3 of material. 1.0 hp/in3/min = 2.73 J/mm3.3. Listed above, average values of unit power required in turning are valid for sharp high-speed steel (HSS) and carbide (WC-Co) tools cutting within the feedrate range of 0.005 to 0.020 inches per revolution and exclude spindle efficiency factor. Average values of unit power required in milling may vary by +/− 10%.4. Values of unit power should be multiplied by a factor of about 1.25 in the case of cutting with dull tools or tools characterized by a negative rake geometry.5. Calculated above, nominal increase in chip temperature is an estimate assuming: (1) constant specific heat of work material across the entire temperature range, (2) no energy losses to work material and tool, and (3) a uniform temperature distribution across chip thickness including the chip/tool contact interface within so-called secondary shear zone.
Table 1also shows how the unit power and energy translate into high temperatures of a machined chip staying in contact with the cutting tool. It is clear that the high-energy materials and cutting conditions require tool grades retaining hardness at the highest temperatures—hard but brittle grades of cemented carbides (WC—Co) and, ideally, advanced non-metallic tool materials that offer an ultimate level of hardness at the cost of low rupture strength and fracture toughness.
Table 2 below outlines the typical values of traverse rupture strength (TRS) and fracture toughness (K1c) of the major groups of tool materials.
TABLE 2Selected Properties of HSS, Carbide andAdvanced Tool Materials - Cermets,Ceramics and Diamond(2)TraverseFracturerupturetoughnessTool materialstrength (MPa)(K1c) MPa m1/2Al2O3500-7002.5-4.5Al2O3—TiC600-8503.5-4.5Al2O3-1% ZrO2700-9005-8Al2O3—SiC550-7504.5-8  SiAlON700-9004.5-6.5Si3N4 100-10001.5-8  SiC550-8604.6Polycryst. CBN (PCBN) 800-1100  4-6.5Polycryst. Diamond (PCD) 390-15506-8TiC—TiN—WC—TaC—Ni—Co—Mo13608.5(C7-C8/C3-C4 class)97WC-3Co (with alloying additions)1590971WC-12.5TiC-12TaC-4.5Co138084WC-16Co (straight cemented3380  10-13.5carbide grades)High speed steel M42 (CPM grade)4000
Comparing to the traditional high-speed steel (HSS) and tougher grades of cemented carbides containing more cobalt binder, the advanced, non-metallic tool materials are significantly more brittle, i.e., sensitive to irregularities in stress loading, irregularities in thermal loading or cooling and thermal stress shocking. Tools with a TRS value of less than 3 GPa (3000 MPa) and a K1c value of less than 10 MPa m1/2 are considered brittle and prone to rapid fracturing under high-energy cutting conditions. Thus, the machining community is aware of the necessity of either avoiding the use of conventional cutting fluids when machining with these brittle tool materials or, if it is possible and practical in a given cutting operation, using the brittle tool materials with extreme care by a complete and uniform flooding of the tool, chip, and contact zone.
For example, numerous publications and tool manufacturer recommendations alert machining operators to the problem of reduced life of ceramic tools on contact with conventional cutting fluids. Despite the inherent deficiencies, e.g., overheated workpiece, reduced dimensional accuracy, or risk of chip fires, dry machining is recommended if hard but brittle tools are used. P. K. Mehrotra of Kennametal teaches in “Applications of Ceramic Cutting Tools”, Key Engineering Materials, Vol. 138-140 (1998), Chapter 1, pp. 1-24: “the use of coolants is not recommended when these tools are used to machine steels due to their low thermal shock resistance”. R. C. Dewes and D. K. Aspinwall (“The Use of High Speed Machining for the Manufacture of Hardened Steel Dies”, Trans. of NAMRI/SME, Vol. XXIV, 1996, pp.21-26) tested a range of oxide and nitride tools including: 71% Al2O3—TiC, 75% Al2O3—SiCw, 50% CBN—AlB2—AlN, 50%-TiC—WC—AlN—AlB2, 80% CBN—TiC—WC, as well as 95% CBN—Ni/Co. They found that the use of a conventional cooling fluid applied by flooding or spraying resulted in the reduction of tool life by more than 95% except for the whisker reinforced alumina, for which the life was shortened by about 88%. Similar test results showing a dramatic tool failure by brittle chipping on contact with cooling fluid have been published for PCBN cutting inserts by T. J. Broskea et al. of GE Superabrasives at MMS Online (www.mmsonline.com/articles) and by others elsewhere.
Table 3 below represents typical machining conditions recommended in the prior art for a range of work materials and tool materials. While different combinations of depth of cut (DOC), feedrate (F), cutting speed (Vc), and unit power (Pc), lead to high or low total power levels (P), the most important value characterizing high-energy cutting and critical to tool life is the power flux (Pf), which is calculated by dividing P by the cross-sectional area of an undeformed chip (a product of DOC and F).
TABLE 3EXAMPLES OF MACHINING CONDITIONS RECOMMENDED IN PRIOR ART FOR A RANGE OF CUTTING,VARIABLES, INCLUDING WORK MATERIALS, WORK HARDNESS LEVELS, AND TOOL MATERIALSDepthRecommendedWork MaterialAssumed: UnitPowerWorkof cut,Feedrate,Cutting Speed,RemovalPower inTotalFlux.MaterialTool Type andDOCFMedium Value,Rate, MRRCutting, PcPower,Pf [hp/Work MaterialHardnessMaterial[inches][inch/rev]Vc [feet/min][in3/min]in3/min]P [hp][kW/mm2]Carbon Steel,150 HBindexable carbide,0.1500.02049017.60.610.23.91020 gradeC-6 (P20)Carbon Steel,150 HBHSS, M2-M30.1500.0151203.20.61.91.01020 gradeH13 Tool Steels,48-50 HRCindexable carbide,0.1500.0101502.71.23.22.5Q&TC-8 (P01)H13 Tool Steels,48-50 HRCindexable carbide,0.3000.0151206.51.27.82.0Q&TC-8 (P01)High-carbon Alloy52-54 HRCindexable carbide,0.1500.0051151.01.61.72.6of Toot SteelsC-8 (P01)Cold Work Too58-60 HRCPCBN (DBC50)0.0120.0044900.33.00.820.4SteelAustenitic St.135-185 HBindexable carbide,0.1500.02035012.60.810.13.9SteelsC-2 (K10/M10)Austenitic St.135-185 HBCold-pressed0.1500.01090016.20.813.010.0SteelsAlumina, ceramicAustenitic St.cold drawnindexable carbide,0.1500.0153008.10.97.33.7Steelsto 275 HBC-3Austenitic St.cold drawnHSS, T15-M420.1500.015802.20.91.91.0Steelsto 275 HBTi-6Al-4V ELI310-350 HBindexable carbide,0.1500.0081952.81.43.93.8C-2 (K10, M10)Ti-6Al-4V ELI310-350 HBHSS, T15-M420.1500.010601.11.41.51.2NOTES:CUTTING POWER, POWER FLUX, AND VELOCITY INDEX ARE ESTIMATED FROM DATA IN TABLE 1.REFERENCES FOR MACHINING CONDITIONS - IAMS AND ASM LISTED IN TABLE 1.Power Flux = Total Power/DOC/F1 hp/in2 = 1.15 W/mm2
The representative examples in Table 3 are not intended to be an exhaustive list. Persons skilled in the art will recognize that numerous other conditions are possible that would result in similar patterns.
High values of power flux indicate the magnitude of potential upset in thermo-mechanical tool loading or irregularity in tool cooling. Only the HSS tools and certain cemented carbide tools operate under the range of cutting conditions where these process irregularities can be neglected. Being a product of cutting speed and unit power, power flux indicates whether a given set of machining conditions leads to a high-energy cutting situation. If a cutting speed is selected for a given tool, depth of cut, and feedrate, which is higher than the cutting speed recommended by the tool manufacturer, and/or the work material requires unit cutting power exceeding 1 hp/in3/minute, the resultant power flux value exceeds the conventional power flux value and the operation may be classified as high-energy cutting.
Although the machining industry has strong economic incentives to enhance cutting operations within the high-energy range, it is limited by tool overheating, high power flux values, and inability of removing cutting energy from tools in a uniform manner required to prevent rapid failures. All tool materials, including HSS, carbides, and refractory ceramics, have one thing in common—as the temperature of the tool material increases, the tool material softens and may develop localized, internal stresses (due to thermal expansion, especially if compounded with limited conductivity), as described by E. M. Trent and P. K. Wright in “Metal Cutting”, 4th Ed., Butterworth, Boston, Oxford, 2000, and the ASM Handbook on “Machining, Ceramic Materials”. This poses limits on workpiece hardness, cutting speed, and power flux during machining. With conventional machining methods, the industry is unable to cope with the cooling problem while satisfying the other needs enumerated above. Other problems facing the machining industry include significant environmental and health related problems associated with the conventional cutting fluids and coolants presently used in the industry. For example, carbon dioxide (CO2), a commonly used industrial coolant, is a greenhouse generator. Also, since CO2 is denser than air it presents a potential asphyxiation concern. In addition, CO2 also has the potential to cause acid corrosion, since it is soluble in water. Freons and freon substitutes, some other commonly used coolants, also are greenhouse generators and ozone depleters. These substances also are explosive and/or toxic when heated on contact with red-hot solids. Other coolants which can be explosive include hydrocarbon gases and liquified ammonia. Coolants such as cryogenic/liquified air with oxygen in it can result in chip fires.
There exists a relatively large body of prior art publications pertaining to cryogenic cooling of tools, including: WO 99/60079 (Hong) and U.S. Pat. No. 5,761,974 (Wang, et al.), U.S. Pat. No. 5,901,623 (Hong), U.S. Pat. No. 3,971,114 (Dudley), U.S. Pat. No. 5,103,701 (Lundin, et al.), U.S. Pat. No. 6,200,198 (Ukai, et al.), U.S. Pat. No. 5,509,335 (Emerson), and U.S. Pat. No. 4,829,859 (Yankoff). However, none these publications nor the other prior art references discussed herein solve the problems discussed above or satisfy the needs set forth below.
U.S. Pat. No. 5,761,974 (Wang et al.) discloses a cryogenically cooled cap-like reservoir placed at the top of a cutting tool, as shown in FIG. 1A herein (corresponding to FIG. 1 of Wang et al.). Wang's method and apparatus provides for uniform and stable cooling, except that the reservoir requires dedicated tooling and repositioning if depth of cut and/or feedrate are changed during cutting operations. Such requirements and limitations are cost-prohibitive and unacceptable in the industrial machining environment.
U.S. Pat. No. 5,901,623 (Hong) discloses a cryogenic fluid spraying chip-breaker which is positioned adjacent the rake face for lifting a chip from the rake face after the chip is cut from the workpiece. See FIGS. 1B and 1C herein (corresponding to FIGS. 3 and 7B of Hong). Hong's method does not provide for uniform cooling of the entire cutting tool, which is desired in the case of hard but brittle tools used in high-energy cutting operations.
Moreover, Hong's chip-breaking nozzle requires dedicated tooling and repositioning if depth of cut and/or feedrate are changed during cutting. Such requirements and limitations are cost-prohibitive and unacceptable in the industrial machining environment.
U.S. Pat. No. 3,971,114 (Dudley) discloses a cryogenic coolant tool apparatus and method in which the tool is internally routed, the internal passage is thermally insulated, and the coolant stream is jetted at a precise angle at the interface between the tool edge and the workpiece so that the chip cutting from the workpiece does not interfere with the stream.
See FIGS. 1D and 1E herein (corresponding to FIGS. 2 and 3A of Dudley). This method also does not provide the desired uniform cooling of hard but brittle cutting tools used in high-energy cutting operations. Moreover, it requires an involved, dedicated tooling. This requirement is cost-prohibitive and unacceptable in the industrial machining environment.
U.S. Pat. No. 5,103,701 (Lundin, et al.) discloses a method and apparatus for the diamond machining of materials which detrimentally react with diamond cutting tools in which hard but brittle tools, which improves tool life in cutting operations characterized by power flux values exceeding the common values recommended for conventional machining processes by tool manufacturers, tool suppliers, and technical authorities recognized within the machining industry.
It is further desired to have an apparatus and a method for cooling such cutting tools that increases work material cutting speeds and/or productivity, both of which are limited by the lifetime (and costs) of cutting tools, inserts, and tips.
It is still further desired to have an apparatus and a method for machining a workpiece which improves safety and environmental conditions at workplaces by using a cryogenic coolant to cool cutting tools, thereby eliminating conventional, emulsified cutting fluids and/or oil mists.
It is still further desired to have an apparatus and a method for machining a workpiece which improves safety and environmental conditions at workplaces by minimizing the risks of chip fires, burns and/or chip vapor emissions while using an environmentally acceptable, safe, non-toxic and clean method of cooling cutting tools.
It is still further desired to have an apparatus and a method for machining which reduces production costs by elimination of workpart, workplace, and/or machine cleaning necessitated by the use of conventional, emulsified cutting fluids and/or oil mists.
It is still further desired to have an apparatus and a method for machining which provides for effective cutting of work materials that cannot tolerate conventional, emulsified cutting fluids and/or oil mists, such as medical products or powder-metallurgy parts characterized by open porosity.
It is still further desired to have an apparatus and a method for cooling cutting tools, an apparatus and a method for controlling cooling of cutting tools during cutting operations, and an apparatus and a method for machining a workpiece, which overcome the difficulties and disadvantages of the prior art to provide better and more advantageous results.